The invention pertains to guided high-intensity focused ultrasound (HIFU) therapy. An ultrasonic treatment device, i.e. a HIFU therapy module, directs the ultrasonic irradiation towards a target zone of a subject of interest, which is usually an alive being, more typically a human being.
Interstitial and intraluminal ultrasonic irradiation devices frequently comprise a mechanically movable transducer, which enables a movement of a beam path of the ultrasonic irradiation. An exemplary application is the treatment of prostate cancer with a rotatable intra-urethral transducer. Accordingly, the transducer is movable so as to control the position within a target zone where HIFU energy is deposited. An actuator is provided to move the transducer. The actuator is controlled by a control unit to move, i.e. to rotate, the transducer, so that a trajectory of HIFU energy depositions is generated. Accordingly, the entire target zone can be heated by the HIFU module as required for the treatment.
Diagnostic imaging like magnetic resonance imaging (MRI) is becoming more important in the area of therapy. In particular, diagnostic imaging is used to obtain temperature information of the target zone to monitor the heating of the target zone. This increases the accuracy of the treatment. Alternatively, also ultrasonic devices can be used as imaging devices in this area.
In current ultrasonic treatment systems comprising the diagnostic imaging device and the ultrasonic treatment device, the diagnostic imaging device is used for providing temperature information in the direction of the beam path of the ultrasonic irradiation. I.e. the diagnostic imaging device is configured to monitor the temperature in the area where the HIFU depositions, also referred to as sonications, are made. A control unit of the ultrasonic treatment device is configured to control the activation at the current target zone on the basis of the measured temperature. The heating is achieved by performing simultaneous sonication and displacement of the beam direction. This is for example proposed in the article “Method for MRI-guided conformal thermal therapy of prostate with planar transurethral ultrasound heating applicators” Rajiv Chopra, Mathieu Burtnyk, Masoom A Haider and Michael J Bronskill, Phys. Med. Biol. 50 (2005) 4957-4975. In particular, it is proposed to use the above interstitial and intraluminal ultrasonic irradiation unit, with an acoustic power and a rotation speed proportional to the difference between the targeted temperature and temperature measured along the sonication direction at the targeted location. This feedback temperature control deliver acoustic power until the target temperature is reached along the beam path at the current target location. However sonications along each direction overlap to some extent due to the width of the ultrasonic beam. Furthermore, the effect of thermal diffusion may result in a further heating of an area, when the ultrasonic beam does not cover this area anymore. As consequence for a specific beam path orientation, an additional temperature rise is taking place at this location once the beam is moved ahead. The heating in a location is completed only once the ultrasonic beam has moved sufficiently away from this location. As consequence the final temperature at the target contour overpass the targeted temperature by a value not predicted.
Accordingly, the control unit decides which sonication parameters such as acoustic power, ultrasound frequency and movement speed are to be applied based on the current temperature within the area of the current HIFU depositions. This requires an accurate calibration of the ultrasonic irradiation unit, in particular in respect to the direction of a beam path of the ultrasonic irradiation. Furthermore, due to beam parameters like beam width, some areas of the subject of interest can be exposed to a dose, which is higher than required. This extends the time required for the treatment of the subject of interest and can lead to undesired treatments of areas of the subject of interest outside the target zone.